Concentric drivers: Pros and Cons?



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One drawback of the design is the production line difficulty in mating the two drivers, and in replacing or reconing the woofer. Another drawback is that the low frequencies tend to modulate the high frequencies, causing greater  intermodulation distortion . [8] The Tannoy style of coaxial, with the woofer forming part of the high frequency horn, had greater intermodulation distortion. Designs similar to the Altec 604 have further problems with diffraction of the low frequencies around the central horn, and with rearward emanations from the horn body reflected forward by the woofer out of time with direct sound. All of the problems with sound waves tend to increase with  sound pressure  level, causing significant shifts in tone as the loudspeaker changes volume. [9]

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https://en.wikipedia.org/wiki/Coaxial_loudspeaker

Modulation of the HF energy by the accompanying LF energy is on paper notable. But in my opinion based upon my experience with my so designed Seaton Cat12Cs, that in actual use, the pros of the coaxial approach render any previous theoretical reservations a moot point.


Due to inherent high sensitivity of the driver, combined with other design choices, the 8" nominal mid-band driver essentially has little to no visible excursion in HT use. That said, concern of audible modulation of the HF by the 8" cone, just doesn't really get off the ground ... especially relative to the vast array of acoustic low hanging fruit needing attention in most Home Theaters.


In the Cat12C, the 8" driver only covers down to approx. the 180hz-200hz range, blending to the flanking 12"s from there. So the excursion is absolutely kept to a minimum, and is all but removed from the pass-band related to the theoretical concern about modulation.



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Originally Posted by FOH  /t/1517946/concentric-drivers-pros-and-cons#post_24365493


Modulation of the HF energy by the accompanying LF energy is on paper notable.

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True, but the same thing occurs with drivers that aren't coaxial, it's not like the low frequency and high frequency wave fronts are isolated from each other. The main issues with coaxials are diffraction and time align. The high frequency output deals with diffraction off the woofer frame, while the acoustic center of the high frequency diaphragm in most designs is well behind that of the woofer. The diffraction issue tends to be no worse than the diffraction that occurs with multiple conventional drivers, while the time align can be addressed in the crossover. An expert in crossover design would have no problem with it.



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Originally Posted by FOH  /t/1517946/concentric-drivers-pros-and-cons#post_24365865


Bill, I'm refuting the importance, not pointing it out. You quote my post as if I'm suggesting it's an issue in use, quite the contrary.

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I wasn't, just that those who suggest that the potential for IMD with a coax tend to ignore that the same will theoretically happen with separate drivers, and it doesn't cause a problem with those either. In the latter case it's not the dispersion pattern of the tweeter that would cause it, it's the woofer, which will be at least 180 degrees at the lower end of its pass band. Time align is important in two ways. The most obvious is a response dip if it results in an out of polarity situation near the crossover frequency. Less obvious is the perceived loss of highs when the lows arrive at the ears first. Both are easily overcome with either a well designed crossover or a DSP.